GRADING AND SCREENING
BY EDWARD S.
Definitions. — This and the succeeding two sections deal with the differentiating of mixtures of materials, solids, liquids and gases by mechanical means or machinery to the end of obtaining one or more portions of the mixed masses in a form useful in the arts. For the purposes of general definition mixtures of solids, gases and liquids will be considered a mass in entirety, even though the separation for example of a solid from a gas could scarcely be denominated a differentiation. The word differentiation is used here in the sense of splitting up a more or less homogeneous mass into components having but slight physical differences from the parent mass. Such differences while slight from the physical sense are often very great in the commercial sense. The separation of the " outside'7 and the "thickness" from the "belly" or portion next to the tree of the bark of Quercus Ilex, a problem the real solution of which still awaits the inventor, makes wood, waste and worthless, and the article of commerce known as powdered or granular cork a commodity of great value.
It is necessary at the outset to distinguish between devices which split or separate a mass into two parts one part being usable and the other being either worthless as in the example cited above or as is most usually the case capable of being further subjected to processing to get an additional yield of usable part, and devices yielding more than two products out of the original mass. In the preparation of the natural amorphous graphite from the Santa Maria mine, Sonora, the raw material after grinding passes to separators of the Raymond type. These yield impalpable graphite and coarse graphite which is returned to the grinding machines. The process is continuous. In the abrasives industry various sized sands are prepared for gluing to paper and cloth by batteries of screens and furnish an example of a device yielding a plurality of products.
Grading. — The definition of this word is "sorting out or arranging in order according to size, quality, rank, degree of advancement, etc." While open to objection this term is the most comprehensive to be found for the operations of assorting or sorting, blowing, classifying, screening and sifting. That these terms are used indiscriminately by inventors a reference to the Patent Office Gazette will disclose.
For the subject matter of this chapter there will be scant necessity for mentioning the terms assorting and sorting. These terms should be used for indicating the operation of removing from a mass of units usually by hand some which are strikingly different from the balance as in the operation of hand sorting ore from waste. The term assorting should be used to indicate the grading of material from coarse to fine by the action of streams and bodies of water. The term sizing has been omitted not because it is not good usage as a synonym for screening but to avoid confusion with the operation of giving surfaces a coat of size or gelatinous wash.
1 Consulting engineer, 409 Boston Building, Denver, Colo.